A significant economic factor in operating aircraft today is the cost of fuel. As a consequence, aircraft designers and manufacturers are constantly striving to improve the overall fuel efficiency. One way to increase fuel efficiency, as well as overall airplane performance, is to reduce the structural weight of the airplane. Since aluminum alloys are used in a large number of the structural components of most aircraft, significant efforts have been expended to develop aluminum alloys that have higher strength-to-weight ratios than the alloys in current use, while maintaining the same or higher fracture toughness, fatigue crack growth resistance, high cycle fatigue resistance and corrosion resistance.
For example, one extrusion alloy currently used as stringers on the lower wing skins of some commercial jet aircraft is alloy 2024 in the T3511 temper. Alloy 2024-T3511 has a relatively high fracture toughness, good high cycle fatigue resistance, very high resistance to fatigue crack growth, and adequate strength and corrosion resistance. Another currently available alloy sometimes used on commercial jet aircraft for upper wing applications is alloy 7075-T6511. Alloy 7075-T6511 is stronger than alloy 2024-T3511; however, alloy 7075-T6511 is inferior to alloy 2024-T3511 in fracture toughness and fatigue crack growth resistance. Thus, the higher strength-to-weight ratio of alloy 7075-T6511 often cannot be used advantageously without sacrificing fracture toughness and/or fatigue performance of the component on which it is desired to use the alloy. Likewise, other currently available alloys in their various tempers, for example, alloys 7175-T6511, 7175-T76511, 7175-T73511, 7050-T76511, 7050-T74511, and 2024-T8511, although sometimes exhibiting good strength or fracture toughness properties and/or high resistance to stress corrosion cracking and exfoliation corrosion, do not offer the combination of improved strength, fracture toughness and fatigue properties over alloy 2024-T3511 for lower wing applications. Thus, with currently available alloys in various tempers, it is usually impossible to achieve weight savings in aircraft lower wing structural components presently fabricated from alloy 2024-T3511 while maintaining fracture toughness, fatigue crack growth resistance and corrosion resistance at or above the current levels.
It is therefore an object of the present invention to provide an aluminum extrusion alloy for use in structural components of aircraft that has a higher strength-to-weight ratio than the currently available alloy 2024-T3511. It is a further object of the present invention to provide this aluminum alloy extrusion with improved fatigue and fracture toughness properties while maintaining stress corrosion resistance and exfoliation corrosion resistance at a level approximately equivalent to that of alloy 2024-T3511.
The microstructure of the alloy is important to obtaining the desired strength properties. It is desired to produce a 2XXX alloy having higher strength than is currently available. In addition, it is desirable to extrude the alloy as quickly as possible for higher productivity.
Accordingly, it would be advantageous to provide a 2XXX alloy having a substantially unrecrystallized structure.
The primary object of the present invention is to provide a method and 2XXX alloy having a substantially unrecrystallized structure.
Another objective of the present invention is to provide a method of extruding a 2XXX alloy having a substantially unrecrystallized structure.
Yet another object of the present invention is to provide a method of extruding a 2XXX alloy having a substantially unrecrystallized structure which permits the use of higher extrusion speeds than is currently used for 2XXX alloys.
Another object of the present invention is to provide 2XXX alloy with improved extrusion press productivity without decreasing the commercial quality of the product that is being extruded. The commercial quality of the extruded product is evaluated in terms of tensile and yield strengths and grain structure.
Yet another object of the present invention is to provide a 2XXX aluminum alloy which can be extruded at the highest possible extrusion speeds without loss of extruded product due to physical defects.
Still another objective of the present invention is to provide a 2XXX aluminum alloy which can be extruded at the highest possible extrusion speeds for a wide variety of shapes and sizes.
These and other objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.